Heightened security concerns worldwide have greatly increased the need for rapid and accurate detection and classification of controlled substances. Detection of controlled substances, with greater certainty, is especially critical at points of entry into countries, transportation hubs, sensitive facilities (e.g., nuclear power plants), and buildings. Securing airports from controlled substances such as explosives, nuclear material, drugs, pathogens, food and their component parts has been complicated by the large volume of luggage moved through airports each day. Currently, luggage is subjected to visual screening using x-ray and/or similar detection devices. This system relies on thousands of individual inspectors to detect all controlled substances and contraband and their component substances despite the efforts of individuals to disguise and hide such materials. However, detection and classification of explosives and certain other controlled substances, and their components, are not amenable to visual inspection either directly or through the use of x-ray machines. Furthermore, previous systems for detecting and classifying explosives and controlled substances are large, very expensive to operate, difficult and expensive to maintain, compromised in terms of speed and detection sensitivity, and result in a high rate of false positive detections.
Various detection systems have been developed to address the need for mechanized detection of explosives and controlled substances. However, to be effective, such detection systems must employ powerful sources, which are quite large and emit potentially dangerous radiation. Attempts to shield workers and the general public from these radiation sources, while providing a reliable detection system of a sufficiently small size to have practical application, have not previously been successful. Accordingly, the radiation sources and the enclosures needed to shield operators and/or the general public from harmful radiation which have been developed to date are simply impractical for use in many facilities including airports and other sensitive locations.
U.S. Pat. No. 5,144,140 (“the '140 patent”) is directed to an analyzer for detecting explosives having at least one source and several detectors having substantially the same spacing from the source. Col. 2, lines 3-5. According to the '140 patent, it is known that when nitrogen, a prime component of explosives, is irradiated with neutrons, it emits gamma rays at predetermined frequencies. Col. 1, lines 14-60. However, since clothing may contain nitrogen at levels that are not significantly different from explosives, analyzers may not be able to distinguish between clothing and explosives. Id. According to the '140 patent, optimizing the distance between the detector and source makes it possible to detect explosives in luggage to a “sufficiently high sensitivity.” Col. 1, line 64-col. 2, line 2.
U.S. Pat. No. 5,076,993 (“the '993 patent”) discloses the use of high energy neutrons in systems to detect contraband to avoid the use of a large number of gamma detectors and to permit sequential interrogating of small volume elements of the object. U.S. Pat. No. 5,098,640 discloses the use of fast neutrons (e.g. 14 MeV) to induce prompt gamma rays from a target object. U.S. Pat. No. 4,882,121 also discloses the use of a 14 MeV neutron generator to irradiate an object suspected of containing an explosive to generate fast gamma photons which are analyzed to determine the nitrogen/oxygen ratio in the object. The ratio is compared to the ratio associated with the explosive to determine if the object contains an explosive. U.S. Pat. No. 5,606,167 discloses the use of a single neutron source to develop a total neutron cross-spectra of an object to identify elements of explosives.
None of the prior systems for detecting explosives and controlled substances are designed to fit in an practically-sized enclosure, operate at a sufficiently low power level, and emit sufficiently low radiation levels to make the same suitable for use in public places, such as airports. Also, although the prior systems are capable of detecting particular explosives and substances, they do not reliably classify the data received to provide an indication of the type of substance detected.